High-sensitivity few-mode long-period fiber grating refractive index sensor based on mode barrier region and phase-matching turning point

This paper presents a novel few-mode long-period fiber grating (FM-LPFG) refractive index sensor, which is based on the high sensitivity of mode barrier region (MBR) and dual-peak resonance near the phase-matching turning point (PMTP) to surrounding refractive index (SRI). The response characteristics of the effective refractive indices of cladding modes with increasing overlay thickness are analyzed and the FM-LPFG sensor can operate in the MBR by choosing the appropriate overlay thickness. Then, a certain grating period is determined to make the coated FM-LPFG work at the PMTP, and the dual-peak resonance characteristics of the FM-LPFG near PMTP with different cladding modes are analyzed. The response characteristics of the dual-peak wavelength spacing variation of FM-LPFG high-order cladding modes to external environment are further studied. The results show that the sensitivity of the FM-LPFG sensor to SRI can be available to 18930nm/RIU when the overlay thickness is located in the MBR and the dual peaks are close to PMTP. Compared to traditional FM-LPFG sensors, the refractive index sensitivity of the sensor is effectively improved by one order of magnitude. What is more, the sensor is insensitive to temperature, which avoids the cross-sensitivity in the process of the refractive index measurement.